This invention relates to a method for forming a solder bump interconnection for attaching an integrated circuit component to a printed circuit board. More particularly, this invention relates to such method in which a solder deposit is applied to a consumable path on the printed circuit board and is drawn along the path onto a terminal pad during reflow to complete the interconnection.
In the manufacture of a microelectronic package, it is known to mount an integrated circuit component to a printed circuit board by a plurality of solder bump interconnections that not only physically attach the component to the board, but also electrically connect the component to the board for conducting electrical signals to or from the component for processing. For this purpose, the printed circuit board comprises a circuit trace, typically formed of copper, which features terminal pads at which the interconnections are made. The component includes a series of metal bumps, typically formed of a solder. The component is superposed onto the board to form an assembly such that each solder bump rests against a corresponding terminal pad. The assembly is then heated to reflow the solder onto the terminal pads. Alternately, solder may be deposited onto the terminal pads and reflowed to bond to the bumps, without necessarily reflowing the bumps. In any event, upon cooling, the solder resolidifies to complete the interconnections.
In the formation of a strong solder bond, it is essential that the molten solder flow into intimate contact with the faying surface during reflow, which phenomena is referred to as wetting. In the absence of wetting, surface tension tends to cause the molten solder to bead up without spreading onto or adhering to the surface.
In conventional practice, the solder is reflowed by heating the entire assembly, for example, within an oven. The oven temperature is typically set as much as 40.degree. C. to 600.degree. C. higher than the solder melting temperature to accelerate melting and minimize the time at the elevated temperature, as well as assure reflow at all interconnections despite temperature variations within the oven. In designing a package, it is necessary to select a solder alloy having a reflow temperature suitably low to avoid damage to other electronic features of the assembly. However, higher temperature solder alloys may have mechanical or other properties that are optimum for a particular package. Therefore, there exists a need for a method for reflowing solder to complete a solder bump interconnection that may be carried out without exposing the entire assembly to high temperatures, so as to permit the use of higher temperature solder alloys without damage to other features of the package.